The term “pre-ignition” refers to a process that results in an uncontrolled and premature ignition of the fuel-air mixture towards the end of the compression phase, especially in Otto engines with a high compression ratio or with supercharging, without an ignition spark having already been generated by the ignition system. A glow ignition is a special sub-type of pre-ignition that is caused by a hot component in the cylinder and that takes place without a knocking level. The rise in temperature caused by the pre-ignition or by the glow ignition, and the increase in the pressure are intensified by the fact that the compression in the combustion chamber is not yet complete. This results in extreme temperature and pressure values in the combustion chamber, which can ultimately lead to the destruction of engine components.
In order to detect the pre-ignition, normally a signal from the knocking sensor is used in order to detect the high-frequency vibrations that usually accompany the pre-ignition.
German patent specification DE 10 2007 024 415 B3 describes a method for detecting a pre-ignition of an externally ignited internal combustion engine having at least one cylinder. According to the method, the rotational speed of the crankshaft during a first time interval is measured during operation of the internal combustion engine. Moreover, a knocking signal is detected by means of a structure-borne sound sensor during a second time interval during operation of the internal combustion engine. Glow ignition of the cylinder is detected when the rotational speed of the crankshaft has slowed down relative to a comparative value and when a knocking combustion is detected on the basis of the knocking signal.
A drawback of this method is that no reliable glow ignition detection is carried out. In particular, detection methods that are based on the use of the knocking signal cannot detect glow ignition since the glow ignitions often occur without any high-frequency components.